1. Field of the Invention
The present invention relates to a diode linearizer that improves distortion characteristics of a high-frequency amplifier.
2. Background Art
In recent years, research and development into transistors using a nitride semiconductor (e.g., GaN) are being actively carried out, and an application example thereof is a high output amplifier for communication. Amplifiers using GaN are enabled to have higher output than conventional compound semiconductors (e.g., GaAs). However, there is a problem that particularly an AMAM characteristic deteriorates due to GaN-specific soft compression in which a gain slowly decreases from low input power.
There are linearizers using a diode as analog predistortion that compensates for the AMAM characteristic. Among such linearizers, parallel diode linearizers have a simple circuit configuration, are smaller and less power consuming (e.g., see Kazuhisa Yamauchi, Kazutomi Mori, Masatoshi Nakayama, Yasuo Mitsui, and Tadashi Takagi, “A Microwave Miniaturized Linearizer Using a Parallel Diode with a Bias Feed Resistance,” IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 45, NO. 12, DECEMBER 1997 and Kazuhisa Yamauchi, Masatoshi Nakayama, Yukio Ikeda, Hiromasa Nakaguro, Naoto Kadowaki, and Takahiko Araki, “AN 18 GHZ-BAND MMIC LINEARIZER USING A PARALLEL DIODE WITH A BIAS FEED RESISTANCE AND A PARALLEL CAPACITOR,” 2000 IEEE).
When a linearizer is incorporated in a GaN-based amplifier MMIC using a SiC substrate, a smallest possible circuit is preferred because epitaxial substrates are expensive. Since parallel diode linearizers require only a simple configuration, they are circuits best suited to a GaN-MMIC.
Parallel diode linearizers are normally designed to have reverse characteristics of AMAM and AMPM of amplifiers connected upstream or downstream thereof. The AMAM and AMPM characteristics of a linearizer vary depending on a voltage applied to a diode of the linearizer. For this reason, the linearizer is provided with a control terminal that controls AMAM and AMPM, and controls and/or adjusts an applied voltage of the linearizer according to AMAM and AMPM of the amplifier.